Regulation .03 Brakes.

A thorough brake inspection includes the inspection of the inside of the brake assembly. The removal of wheel hubs and drums from large commercial vehicles is difficult because of size and weight problems and extreme care shall be taken to assure they are properly returned to the axle.

A thorough examination of the hydraulic, vacuum, or air systems will normally give a fair idea of general condition of the braking system, especially when augmented by a practical performance demonstration.

Road. Simple tests and visual procedures. Service brake tests shall be conducted on a substantially level, dry, hard, smooth surface road or area that is free from loose material, oil, or grease. Using the service brake only, the stopping ability of the vehicle shall be tested by one of the following methods:

Platform Test. Before attempting inspection by this method, the inspector should be trained and experienced in the use of the equipment and the equipment must have adequate capacity.

Dynamic Brake Inspection. This test is for stations equipped with a roller-type brake dynamometer. The rollers should be dry, smooth and free of oil or grease. The equipment indicates both braking effort and imbalance. Brakes on front and rear axles are evaluated separately.

Tests and visual inspection procedures. Recommended for inspection programs which must accommodate a large volume of vehicles. Results will indicate whether a vehicle has reasonably safe brakes at the time of inspection. The engine shall be running when vehicles are checked with power assisted hydraulic systems. These procedures are applicable to vehicles equipped with hand controls for the physically handicapped.

At least one wheel and drum assembly shall be removed from each axle equipped with brakes. On disc brakes only wheel assembly removal is necessary. Any wheel or drum assembly removal shall be on alternate sides. Always inspect a brake assembly which shows evidence of leaking fluid or grease. Do not remove hydraulic wheel cylinder dust boots for inspection. Leakage can be determined by pushing on the outside of the boot.

Some vehicles have a combination of disc (caliper) type brakes on the front wheels and drum-type brakes on the rear wheels.

On vehicles equipped with disc brakes, some drag can be felt when turning the wheel and tire. This drag is not excessive if the wheel can be turned readily with both hands.

This deceleration as measured in brake tests cannot be used to compute the values in Column 4 because it is not sustained at the same rate over the entire period of the stop. The deceleration increases from zero to a maximum during a period of brake system application and brake force build-up. Also, other factors may cause the deceleration to decrease after reaching a maximum. The added distance which results because of a maximum deceleration is not sustained is included in the figures in Column 4 but is not indicated by usual brake testing devices for checking deceleration.

There is a definite mathematical relationship between the figures in Columns 2 and 3. If the decelerations set forth in Column 3 are divided by 32.2 feet per second per second, the Column 2 figures will be obtained (for example, 17 divided by 32.2 gives 52.8 percent). Column 2 is included in the tabulation because certain brake testing devices utilize this factor.

The decelerations as in Column 3 are an indication of the effectiveness of the basic brakes, and as measured in practical brake testing are the maximum braking decelerations attained at sometime during the stop.

The distances in Column 4 and the decelerations in Column 3 are not directly related. Brake system “application and braking distance in feet” (Column 4) is a definite measure of the overall effectiveness of the braking system, being the distance traveled between the point at which the driver starts to move the braking controls and the point at which the vehicle comes to rest. It includes distance traveled while the brakes are being applied and the distance traveled while the brakes are retarding the vehicle.

The distance traveled during the period of brake system application and brake force buildup varies with vehicle type, being negligible for many passenger cars and greatest for combinations of commercial vehicles. This fact accounts for the variation from 20 to 40 feet in the numerical values in Column 4 for the various classes of vehicles.

The deceleration requirement in Column 3 is the same for all classifications of vehicles except for passenger vehicles, not including buses, because brakes on vehicles in the second, third and fourth classifications are all capable with reasonable maintenance of producing the designated deceleration as measured by brake testing devices. A higher deceleration requirement is warranted for passenger cars in view of United States Department of Transportation/Federal Highway Administration test data.

The method of brake system application and braking distance is not to be confused with the distance indicated by the inertia-type decelerometer and other existing brake testers, which often are purported wrongly to measure the vehicle stopping distance.

The pendulum and U-tube decelerometers used for brake testing are instruments scaled to read deceleration or equivalent braking force (sometimes referred to as brake efficiency) in percentages. The principle of the pendulum-type decelerometer is that a pendulum on a vehicle moving at a uniform speed will assume a vertical position. When the vehicle speed is reduced by application of the brakes, the pendulum will swing forward to an angle away from the vertical. The tangent of the angle through which the pendulum moves is directly proportional to the deceleration. Basically, the U-tube fluid-type instrument is a closed glass tube formed in the shape of a U. When vehicle speed is reduced by braking, the inertia of the fluid causes the level in the glass tube to fall. The distance the level of the liquid falls is proportional to the deceleration in feet per second per second, which is read from a scale on the front part of the glass tube.

The vehicle will normally pitch because the lines of action of the inertia and braking forces are different. In order to minimize erroneous responses, the decelerometer, if used, should be put as close to the center of the vehicle as practical.

These types of brake testers measure braking force at each wheel. The effectiveness of the brake testers that measure braking force in the testing of vehicles other than passenger vehicles or light trucks is very questionable.

The dimensions of these brake testers do not permit practical and reliable testing on combination vehicles. Use should therefore be restricted to two-axle vehicles. The simulated road surface of surface of these machines will occasionally exhibit much higher coefficients of friction than is possible on the highway. Excessive braking should therefore be avoided, be avoided, because braking distribution information is distorted.

For drive-on-and-stop test machines, the vehicle is driven on the pads at speeds of 4 to 8 mph. When the brakes are applied at the time the vehicle is moving on the pads, the braking effort at each wheel causes a proportionate movement of the pad against the measuring system. The braking force on the pads is measured by indicating or recording instruments.

The roller-type brake testing machine has powered rollers that turn the individual wheels while the brakes are applied in order to measure the brake force developed.

It measures the distance within plus or minus 3 percent of the values obtained by the “hot-shot” method, which uses an electrically fired gun.

The road speed of the test wheel is measured with a speedometer designed to hold the indicating hand at the speed from which the stop is initiated. The speedometer shall indicate the speed to within plus or minus 2 percent of the speed timed with a stopwatch over a measured mile.

“Brake system” means a combination of one or more brakes and their related means of operation and control.

“Deceleration” means the rate of reduction of the speed of the vehicle, expressed in feet per second per second.

“Equivalent braking ratio” means the percentage ratio of the sum of retarding forces developed by each braked wheel to the “as tested” gross weight of the vehicle or combination.

“Emergency brake system” means a brake system used for retarding and stopping the vehicle in the event of a malfunction in the service brake system (this function may be performed by the parking brake system or by a portion of the service brake system, or by a separate brake system).

“Parking brake system” means a brake system used to hold and maintain a vehicle in a stationary position (a positive mechanical means is employed to hold the brake applied when the vehicle is unattended).

“Pedal reserve” as applied to hydraulic, mechanical, or power-assisted hydraulic brakes, means the amount of total pedal travel left in reserve when the pedal is depressed to the brake-applied position (the purpose of the pedal reserve check is to ascertain the degree of the brake adjustment and to demonstrate satisfactory brake actuating system condition).

“Service brake system” means a brake system used for retarding, stopping, and controlling the vehicle under normal operating conditions.

“Stopping distance” means the distance traveled by a vehicle from the point of application of force to the brake control, to the point at which the vehicle reaches a full stop.